Known pump mechanisms for common rail fuel injection systems have a number of disadvantages. The sliding-type common rail pump mechanism comprises a tappet which slides on a rider. As a result, this mechanism generates a large amount of heat which leads to a decrease in the life of the reciprocating components. Furthermore, to generate higher pressures with this mechanism, larger components are required, which in turn makes it difficult to fit the pump into smaller engines. Another type of common rail pump comprises a roller and shoe running on a single or multi-lobe cam. While this design has the advantage of producing less heat, the pressure capability is limited given the space available in the engine. Also, the rolling inertia tends to impact negatively upon the fatigue strength of the cam and roller when the parts are made larger due to pressure requirements.